The photo CVD process is a main means for the formation of thin film among low temperature processes and restrains the interdiffusion at the surface without damaging the substrate, so that it is for the formation of amorphous silicon films for solar cell (hereinafter referred to as a-Si film), or diamond-like carbon thin films and carbon films having a diamond structure (hereinafter referred to as diamond film) expected as a high functional material having simultaneously high hardness, high thermal conductivity, high insulation property and the like, or thin films of silicon carbide, silicon nitride and silicon dioxide used as or applicable for semiconductor devices.
Since such thin films play an important part in the modern industry, film formation processes represented by PVD process (Physical Vapor Deposition process) and CVD process (Chemical Vapor Deposition process) have been recently developed and applied and practised in various fields.
Among them, photo CVD process wherein a source gas is in the vicinity of a substrate and is subjected to photolysis and then the resulting active chemical species are deposited onto the substrate is important as one of low temperature processes causing no damage to the substrate and inducing no unnecessary interdiffusion at the surface.
As a light source for the photo CVD process, there are generally two kinds of continuous light and discontinuous light (pulse beam).
The former case includes a low pressure mercury lamp, a high pressure mercury lamp, a deuterium lamp, a xenon lamp, an Ar ion laser and the like. These continuous light sources have a drawback that the deposition rate is very slow because a light of absorption range which induces excitation causing photolytic process inherent to the source gas is not emitted, or if a light of a wavelength existing in this absorption range is emitted, the power thereof is weak.
The photo CVD process has a merit capable of forming films in a low temperature process, which is never achieved in the other film forming processes, but there are problems to be solved, such is small deposition rate and poor film quality in the photo CVD process using the continuous light. .Therefore, there have been proposed various methods for solving these problems.
The proposed techniques are as follows.
(1) Method wherein laser beams are focused and irradiated to a reactive gas from two or more different directions to expect local multiphoton dissociation process (Japanese Patent laid open No. 61-96,725).
As shown in FIG. 1, this method is performed by placing a substrate 5 in a reaction chamber 1 provided with a light irradiation window 2, introducing a gas from an inlet 3 and irradiating two laser beams 7, 8 from different directions to a point P on the substrate.
(2) Method of synthesizing a thin film by simultaneously introducing plural source gases into a CVD reaction chamber and simultaneously irradiating lights of plural wavelengths matched with the absorption spectrum of each gas (Japanese Patent laid open No. 60-206,445).
(3) Method wherein lights of two different wavelengths are simultaneously irradiated into a CVD reaction chamber to cause decomposition of a source gas through a light of a first wavelength and successively decompose the resulting decomposed products through a light of a second wavelength (Japanese Patent laid open No. 62-7,122).
(4) Method wherein a laser beam is irradiated in parallel to a substrate surface to cause photolysis of a source gas and another laser beam is irradiated to the substrate to heat the substrate for increasing the deposition rate (Japanese Patent laid open No. 61-30,028) or cause excitation of the substrate for increasing the deposition rate (Japanese Patent laid open No. 61-108,130).
In these methods using the continuous light, the deposition rate is small only with the single continuous light, so that it is intended to improve the deposition rate by adding another continuous light. However, the energy of the continuous light is generally low, so that the expected effect is not obtained by using only the continuous light source.
On the other hand, in the method using a strong discontinuous light source represented by an excimer laser or the like having a high energy, a slightly large deposition rate can be realized when using a source gas having a large absorption cross section, but it is difficult to form a thin film having a good film quality because powder is undesirably produced.
The most serious drawback of the excimer laser is that it is a pulse light source, so that there are disadvantages that the half-width of pulse is not more than 20 ns and the pulse repeating number is restricted. That is, the effective time causing the photolytic reaction actually contributing to the formation of films is very short in a time required for the film formation. For example, when a duration time of one pulse beam is 20 ns, even if the beam is irradiated at 100 Hz for 1 hour, the beam irradiated time is only 7.2.times.10.sup.-3 seconds (20.times.10.sup.-9 .times.100.times.60.times.60=7.2.times.10.sup.-3). Since the beam of high energy is irradiated for a very short time, the degradation of the film quality inclusive of production of powder is brought about. On the other hand, when using a source material of poor photochemical reactivity for controlling the production of powder, the deposition rate lowers and at the same time a greater part of the source gas is discharged as an unreacted state.
Under the same irradiation conditions, the irradiation time of not more than 20 ns and the non-irradiation time longer by 1,000,000 times than the irradiation time, i.e. 10 ms are repeated, so that the concentration of radicals produced largely changes between a pulse and the next pulse and consequently the main elementary reaction is different depending on the time and the reaction occurs at a non-steady state. That is, the preferable reaction for the film formation proceeds for a certain time, while particles are formed for another certain time to mainly cause the reaction degrading the film quality, whereby the degradation of the film quality is caused in the CVD process using the pulse laser beam.
In addition to problems relating to the light irradiating process as mentioned above, the conventional photo CVD processes have problems as mentioned below For instance, if it is intended to produce a-Si film, diamond film or the like by using the conventional photo CVD process based on direct photolysis of the source gas, it is necessary to provide a light source of a wavelength capable of being absorbed by the particular source gas or to prepare a source gas capable of being subjected to a photolysis by the particular light source.
However, even if the light source suitable for the particular source gas is existent, since the decomposition quantum efficiency of the source gas decomposed by this light source is inherent to this gas, it is impossible to promote the film forming reaction exceeding the above quantum efficiency. This is a drawback in the conventional photo CVD process.
For example, if it is intended to produce a-Si film, since SiH.sub.4 having an absorption end of 170 nm is not decomposed by the commercially available lamp light, it is necessary to decompose an expensive source gas of Si.sub.2 H.sub.6 having a shifted absorption end of 200 nm by a light of an excimer laser having a wavelength of 143 nm, resulting in the rise of cost and the degradation of the film quality.
Moreover, the film formation can be also carried out by a combination of a mercury lamp and SiH.sub.4, but in this case, it is necessary to adopt a so-called mercury sensitization process wherein Hg vapor is mixed with the source gas.
That is, a Hg atom is excited by a light from the mercury lamp and collides with SiH.sub.4 molecule to cause energy transfer from the Hg atom, whereby Si-H bonds in SiH.sub.4 are cut to form a-Si film onto the substrate. In the mercury sensitization process, however, Hg atom is included in the film, resulting in a large cause of degrading the film quality of the a-Si film.
On the other hand, if it is intended to produce diamond film, when using CH.sub.4 (absorption end 160 nm) as a source gas, it is not decomposed even if a practicable light source having a shortest wavelength of 193 nm is used alone, but it is first possible to form a film by using, for example, F.sub.2 excimer laser beam of vacuum ultraviolet region (wavelength 157 nm). However, the pulse energy of the laser beam having a wavelength of 157 nm is as small as about 10 mJ/pulse and is about 1/10.about.1/20 as compared with the case of a light having the other wavelength, and also this wavelength is large in the absorption by air and difficult in handling, so that it is unpratical.
Furthermore, when a source gas having a weak or no one-photon absorption process is used to perform the film formation at a practical wavelength of 193 nm, as disclosed in Japanese Patent laid open No. 60-112,697, the decomposition should be carried out by two-photon absorption at a region having an energy density raised by focusing of the beam, so that the film forming area is restricted and the deposition rate becomes small.
Similarly, Japanese Patent laid open No. 60-146,791 has proposed the use of a gas such as acetylene, ethylene or the like as a source gas decomposed at the wavelength of 193 nm, but the use of such gas is unpratical because the deposition rate is restricted.
As mentioned above, the photo CVD process has a merit that it is a low temperature process and a demerit that the deposition rate inherent to photo CVD is small, so that the excimer laser beam having a strong light emission in the ultraviolet region is frequently used in order to overcome this demerit. However, the excimer laser beam is emitted in a series of pulses because of restricting the oscillation mechanism and electric circuits, while there is a limitation in rising its frequency.
Therefore, there are two great disadvantages in the photo CVD process using the pulse laser beam as a light source, which results from the above restriction.
That is, the first case is that the effective time of light irradiation is small and hence the waste of the source gas is large. The second case is that the reaction is rendered into a non-steady state to cause a so-called "disturbance of reaction" and hence the film quality is degraded.
It is an object of the invention to avoid the reduction of decomposition rate of the source gas resulted from an extremely short irradiation time in the film formation through the photo CVD process using a pulse ultraviolet laser of a large power and hence improve the small deposition rate.
It is another object of the invention to increase the deposition rate to thereby improve the film quality by taking a means for minimizing the "disturbance of reaction" resulted from the extreme change of radical concentrations between a pulse and the next pulse.
Here, it is important to attempt the improvement of the film quality by stably and continually conducting photochemical reaction in the vapor phase near to the substrate or on the substrate itself or in the film layer formed on the substrate even in a non-irradiated time between a pulse and the next pulse.
On the other hand, the most fundamental problem in the photo CVD process lies in that the source gas suitable for the film formation is not necessarily adaptable to the practical light source.
That is, the preferable source gas is not necessarily decomposed by absorbing a light from the practical light source, or inversely, there is existent no effective source gas suitable for excitation and decomposition by the light source.
Moreover, even when the proper source gas is decomposed by the practical light source, if the absorption cross section of the source gas for the wavelength is small, photons are hardly absorbed by gas molecules, so that it is disadvantageous for economical reasons. Further, in the process of excitation and decomposition of particular source gas, there is an upper limit of quantum efficiency of decomposition resulted from the combination of elementary reactions, so that the yield exceeding the upper limit is not expected and further developing the efficiency of the process is impossible.
That is, all of the conventional photo CVD processes perform the decomposition of the source gas itself by the light itself, so that they have a problem that the source gas, light source and film forming area are subjected to a great restriction in practical use.
Now, the other object of the invention is to provide a photo CVD process enabling the use of inexpensive source gas which realizes the application of a light source irrespective of the wavelength value of light absorption end of the source gas and the improvement of the film quality and deposition rate.